1. Field of the Invention
This invention relates to methods and systems for imaging of spatial variation of acoustic parameters of an object and particularly gas bubbles and high density scatterers in the object. The methods have applications in a variety of fields with a variety of objects, for example ultrasound imaging of biological tissues and fluids, acoustic imaging of geologic structures, and detection of objects in water with SONAR.
2. Description of the Related Art
Acoustic imaging is used in a variety of applications, such as medical ultrasound imaging of internal organs, SONAR imaging of fish, sea animals and other objects in the sea, imaging of geologic structures for various purposes such as studies of archeological digs and surveillance of oil wells. A wide range of frequencies of the transmitted acoustic pulse are used for different applications, ranging from infrasound for imaging of some geological structures to ˜100 MHz ultrasound imaging of some biological and microscopic structures. Despite this spread of applications and imaging frequencies the imaging methods are very similar for all applications. In this patent we therefore refer to these imaging methods in general as acoustic imaging, whereas a large part of the applications, especially medical applications, will use inaudible ultrasound frequencies from ˜20 kHz to well into the ˜100 MHz range. Where the imaging frequencies are in the ultrasound range, such as medical imaging, we also will use the term ultrasound imaging, not limiting the methods to ultrasound frequencies and medical applications only.
Despite the widespread use of acoustic imaging, current images are noisy, require large skills for the interpretation, and provide limited quantitative information about the objects. This provides problems for differentiation of object structures and estimating quantitative object properties. With SONAR imaging in water it is for example often difficult to differentiate objects close to or on the sea bed, like fish or other sea animals or mines, from the seabed echoes. Similarly, in geologic imaging it can be difficult to determine material compositions of the geologic structures. In medical applications it can be difficult to differentiate structures like a tumor or atherosclerotic tissue from normal tissue. Important reasons for this are described below.
Spatial variations in the linear acoustic properties of the object (mass density and compressibility) are the physical basis for acoustic imaging. However, with large variations of the acoustic properties in complex structures, the following effects will degrade the images:
i) Interfaces between materials with large differences in acoustic properties can give so strong reflections of the acoustic pulse that multiple reflections get large amplitudes. Such multiple reflections are termed pulse reverberations, and add a tail to the propagating acoustic pulse, which shows as noise in the acoustic image.
ii) Variations of the acoustic velocity within the complex object structures produce forward propagation aberrations of the acoustic wave-front, destroying the focusing of the beam main lobe and increasing the beam sidelobes.
The reduced focusing of the beam main lobe by the wave-front aberrations reduces the spatial resolution in the acoustic imaging system. The pulse reverberations and the increase in beam side lobes by the wave-front aberrations, introduce additive noise in the image, which reduces the ratio of the strongest to the weakest scatterer that can be detected in the neighborhood of each other, defined as the contrast resolution in the image. This noise is termed acoustic noise as it is produced by the transmitted acoustic pulse itself. Increasing the transmitted pulse power will hence not improve the power ratio of the signal to the noise of this type, contrary to what is found with electronic receiver noise.
In echocardiography for example, pulse reverberation noise can obscure images of the apical region of the heart, making it difficult to detect apical thrombi, and reduced contraction of the apical myocardium. Further by example, in carotid imaging reverberation noise can obscure detection and delineation of a carotid plaque. Similar to these examples, the pulse reverberation noise limits the detection of weak targets and differentiation of small differences in image contrast in all aspects of acoustic imaging.
2nd harmonic imaging is a method to reduce the image degrading effect of the pulse reverberations in structures close to the acoustic source like the human body wall, because the 2nd harmonic content in the pulse accumulates as a function of depth and is hence very low as the pulse passes the near source structures like the body wall. However, the sensitivity with 2nd harmonic imaging is less (˜−20 dB) than with 1st harmonic imaging, which limits maximal image depth, particularly in dense objects like geologic structures and biological objects like the liver, kidneys, breast, etc, and for blood velocity imaging. For real time 3D imaging one wants a broad transmit beam that is covered with many parallel receive beams to increase volume image rate. Such broad 2nd harmonic transmit beams are difficult to obtain due to reduced 1st harmonic amplitude in broad beams, which produces problems for 2nd harmonic imaging with multiple parallel receive beams used in real time 3D imaging. This is especially true for sparse acoustic arrays where the number of elements that generates the transmit beam is limited.
In medical applications, tissue diseases like tumors and atherosclerosis of an artery wall, affect the acoustic parameters of the tissue, such as the shear modulus, the bulk compressibility, and the acoustic absorption. The variations of these properties are mainly produced by in-growth of foam cells, fat, or connective tissue fiber molecules, but also through segregation of calcium in the tissue. The in-growth of connective tissue increases the acoustic absorption and the shear modulus, the latter producing an increased stiffness to palpation that can be observed by touching the tissue. Much work has been done on estimation of the shear modulus by using ultrasound bulk waves to register the displacement of shear waves in the tissue in methods often referred to as elastography, also referred to as remote ultrasound palpation. However, to date these methods have found limited clinical application, and there is still a great need for improved differentiation of such tissue changes with ultrasound.
In breast tumors, segregated micro-calcifications are today detected with X-Ray mammography, as an indication of a malignant tumor. These micro-calcifications are so small that the scattered ultrasound signal from them is buried in the signal from surrounding tissue, and they are not detected with current ultrasound imaging. Hence, it is a need to improve ultrasound imaging to also detect such micro-calcifications. Micro-calcifications in atherosclerotic plaque also give information about the stability of the plaque and improved imaging of these micro-calcifications are needed.
Several diseases also affect the blood perfusion through the tissue, for example through angiogenesis or necrosis of the micro-vasculature in malignant tumors, or reduced blood flow due to vascular stenosis or thrombosis both in the coronary arteries of the heart and in peripheral vessels. The blood velocities in the micro-vasculature and small vessels are so small that they cannot be detected with ordinary, non-invasive ultrasound Doppler techniques. Ultrasound contrast agents in the form of solutions of small micro-bubbles (diam ˜3 μm) have therefore been developed to improve ultrasound imaging of the micro-vasculature and also to estimate the blood perfusion through the tissue. The micro-bubbles are injected into the blood stream and provide highly increased and nonlinear scattering of the ultrasound from the blood. They hence highly increase the nonlinear scattering from the tissue that contains such micro-bubbles, where in special cases single micro-bubbles can be visualized in dense tissues and provides a potential for molecular ultrasound imaging with tissue specific targeted contrast bubbles. Such micro-bubbles can also provide useful image enhancement when injected into other body fluids, for example the insterstitial fluid to trace lymphatic drainage to sentinel lymph nodes, or in the urinary system for targeted attachment of bubbles to tumor tissue, or other. During decompression in diving and space activities, micro gas bubbles often form spontaneously in the tissue causing decompression sickness, and it is a need for early detection of such gas bubbles to improve decompression profiles and avoid decompression sickness in personnel under such operations, and even to monitor formation of such bubbles as an early warning during activity.
During production in an oil well, one for example wants to monitor changes in the geological structures around the oil wells, for example to monitor the amounts of oil or gas in the sand stone, observe the boundary between gas/oil and water, and observe any structural slides in the neighborhood of the well. The acoustic properties of the structures, and particularly the nonlinear component of the acoustic properties, are influenced by the amount of gas, oil, or water in the porous rock. Acoustic imaging of the structures surrounding the oil well can be done from acoustic transducers in the oil well. Utilizing imaging methods that provide quantitative acoustic data from the object hence allows detections of the amount of gas, oil, or water in the structures surrounding the oil well.
In detection of fish or sea animals or other objects close to the seabed it is often difficult to differentiate between the echoes from the seabed and the object, particularly with side looking beams. The swimming fish or sea animals has a gas filled bladder or lungs that has quite different and nonlinear acoustic properties compared to those of the seabed. These differences in acoustic properties can with methods according to the current invention be used to differentiate overlapping echoes from such objects and the seabed. The methods can also be used to enhance small solid structures, like a mine, on a softer seabed or in soil, similar to detection of micro-calcifications in a tumor.
There is hence a great need for improved acoustic imaging that reduces the image noise, and enhances the image contrast for variations in object properties,
Methods according to the current invention reduce the image noise, and enhance the image contrast for variations in object properties by transmitting dual frequency band acoustic pulse complexes composed of overlapping high and low frequency pulses into the object. Dual frequency band ultrasound pulses have previously been used in medical ultrasound imaging for various purposes, where in M-mode and Doppler [Br Heart J. 1984 January; 51(1):61-9] simultaneous transmission was used of a 3 MHz pulse and a 1.5 MHz pulse with fixed phase relation between the pulses, for optimal M-mode imaging of the heart (3 MHz pulse) and Doppler blood velocity measurements (1.5 MHz pulse) to interrogate cardiac defects. A concentric annular transducer arrangement was used, where the 3 MHz M-mode ultrasound pulse was transmitted and received by the central transducer disc, while the 1.5 MHz Doppler ultrasound pulse was transmitted and received by a surrounding annular element.
The use of dual band transmitted pulses is also described in U.S. Pat. No. 5,410,516, for improved detection of ultrasound contrast agent micro-bubbles. In this patent, simultaneous transmission of two ultrasound pulses with different center frequencies is described, where the scattered pulses from the micro-bubbles contain sums and differences of the transmitted frequencies produced by the nonlinear scattering from the micro-bubbles, and these sum and difference frequencies are used for the detection of the micro-bubbles.
A similar use of dual band pulses is described in U.S. Pat. No. 6,312,383 for detection of ultrasound contrast agent, where the phase between the two bands is changed between transmissions. This can be viewed as a special case of U.S. Pat. No. 5,410,516, where the change in phase of the low frequency pulse can be viewed as a beat between the low frequency and the pulse repetition frequency.
However, although both the last two patents use nonlinear scattering with dual band pulses for detection of contrast agent in tissue, the presented patents both fail to recognize the nonlinear effect of the low band pulse on the forward propagation velocity of the high band pulse, which in the practical situation will limit the suppression of the tissue signal in relation to the contrast agent signal. The patents also do not recognize how the nonlinear scattering from ordinary tissues or other objects can be retrieved. Accumulative nonlinear forward propagation effects will produce similar signal characteristics for the strong, linear scattering from the tissue, as for the local, nonlinear scattering from micro-bubbles and tissues. This effect will mask the local, nonlinear scattering from micro-bubbles and other object parts and limit the contrast to tissue signal power ratio (CTR). Presence of gas and micro-bubbles in a region also heavily increases the forward, accumulative, nonlinear propagation effect and makes the linear scattering from the object beyond such a region highly mask the scattering from gas and micro-bubbles in the object. This phenomenon for example highly affects imaging of contrast agent in myocardium with pulses that passes the ventricle with contrast agent before entering the myocardium, and can for example falsely indicate perfusion in an ischemic myocardium. It will also affect the differentiation between gas and oil past a region with gas in geologic structures.
The current invention differs from the prior art in that it utilizes the nonlinear effect on the propagation velocity for the high frequency pulse by the low frequency pulse, and an understanding of this effect, in the formation of image signals based on the high frequency propagated and scattered signals. This allows a separation of the accumulative nonlinear effect on the signals from the effect of the local nonlinear object parameters, hence allowing estimation of local nonlinear object parameters, which is not possible by prior art. The invention further devices a method for separation of the accumulative effect of acoustic absorption on the signals, enabling the estimation of the local acoustic absorption parameters of the object.